One technology for after-treatment of engine exhaust utilizes .selective catalytic reduction (SCR) to enable certain chemical reactions to occur between NOx in the exhaust and ammonia (NH3). NH3 is introduced into an engine exhaust system upstream of an SCR catalyst by injecting urea into an exhaust pathway. The urea entropically decomposes to NH3 under high temperature conditions. The SCR facilitates the reaction between NH3 and NOx. to convert NOx into nitrogen (N2) and water (H2O). However, as recognized by the inventor herein issues may arise upon injecting urea into the exhaust pathway. In one example, urea may be poorly mixed into the exhaust flow (e.g., a first portion of exhaust flow has a higher concentration of urea than a second portion of exhaust flow) which may lead to poor coating of the SCR and poor reactivity between emissions (e.g., NOx) and the SCR. Additionally, overly mixing and agitating the urea in the exhaust can likewise cause issues, such as increased deposits.
Attempts to address poor mixing include introducing a mixing device downstream of a urea injector and upstream of the SCR such that the exhaust flow may be homogenous. One example approach is shown by Collinot et al. in U.S. 20110036082. Therein, an exhaust mixer is introduced to an exhaust pathway to both reduce exhaust backpressure as exhaust flows though the mixer and increase exhaust homogeneity. The exhaust mixer comprises one or more helicoids which may manipulate an exhaust flow to flow within an angular range of 0 to 30°.
However, the inventors herein have recognized potential issues with such systems. As one example, the mixer introduced by Collinot has a relatively long body and may additionally comprise one or more mixer bodies adjacent to one another. The mixer bodies may vibrate and collide with one another, due to either road conditions or turbulent exhaust flow, which may produce undesired audible sounds and/or prematurely degrade the mixer.
In one example, the issues described above may be addressed by a mixer with an annular mixer pipe located outside of an exhaust passage, and where mixer tubes extend radially inward from the mixer pipe into the exhaust passage. In this way, exhaust gas flowing out of the mixer flows to regions of the exhaust conduit unperturbed by the mixer and increases an overall homogeneity of exhaust gas in the exhaust conduit. Thus, mixing is increased and a composition of exhaust gas throughout the entire exhaust conduit is substantially equal.
As one example, the mixer tubes are misaligned with one another as are inlets and outlets of the mixer. This allows the mixer may intercept exhaust gas along various regions of the exhaust conduit. The mixed exhaust gas flows back into the exhaust passage along its outer periphery. In this way, exhaust gas is drawn from a central portion of the exhaust passage and redirected to an outer portion of the exhaust passage to increase homogeneity.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.